A recent study by the
National Association of Home Builders (NAHB) Research Center
concluded that the national average for hot water usage in
homes is approximately 62 gallons per day or 22,630 gallons
per year.1 To heat all of this water, there
are many choices in water-heating systems. All such systems
serve to raise the temperature of the incoming water to about
120°F (49°C) at the hot water outlets. The heart of the
system is the water-heating unit that may be gas fired,
electric, solar, or energized by other means.
This article specifically
concentrates on the increasingly popular water heaters
variously known as on-demand, instantaneous, or tankless water heaters. Further, only electric units are considered
although gas-fired units are available. After defining these
water heaters, their applications and performance
specifications are covered. Then, they are compared to the
conventional tank-type water heaters in terms of the
advantages and disadvantages. Finally, the article describes
the electrical requirements and the National Electrical
Code rules that apply to these tankless water heaters.
Definition
As
an alternative or booster unit to a tank-type water heater, a
number of products for heating water have been developed that
respond only when hot water is demanded from a tap. Although
these so-called instantaneous or tankless water heaters are
not exactly either instantaneous or tankless, such names are
frequently applied to these units and will be used in this
article. These heaters are termed tankless because they
are characterized by a very small storage capacity, usually
less than a gallon of water. Their capability to heat the
water to temperature very rapidly, within a few seconds to 30
seconds when it is flowing, leads to the description instantaneous.
The tankless water-heaters
can be characterized by their intended application. Units that
serve only one tap or faucet are called point-of-use heaters. Point-of-use heaters are popular in Europe and Asia.
More powerful units that can serve the hot water needs of a
home are called whole-house tankless heaters. This
article mainly considers tankless, whole-house electric
water-heaters as installed in the U.S. and in Canada. The
heaters use resistive heating elements in the water flow
chambers to heat the water. Photo 1 shows a tankless water
heater that is suitable for whole house applications. The unit
is about the size of a large briefcase and weighs just over 20
pounds. Cold water enters from the right-hand inlet and the
heated water flows out of the connection on the left. The
circuit hookup is not shown.
Photo
1. Example of a tankless water heater about the size of a
large briefcase
Applications
Tankless
water heaters are used in various applications where hot water
is used for domestic purposes, such as washing and showering,
and also in space heating installations. For some homes, a
tankless water heater of the proper capacity could serve all
of the domestic hot-water needs. Other home installations with
heavy hot-water demand might require several such heaters or a
combination of a tankless and tank heater. Another design of a
hot-water system is to combine a solar system with a tankless
heater to assure the availability of hot water when solar
energy is not available to heat water.
Space heating applications
for which tankless water heaters are ideal involve using hot
water in a recirculating system to heat rooms in a house. The
heat may be transferred from underfloor pipes through which
the hot water flows or from baseboard heat exchangers.
Many commercial
establishments have installed tankless water heaters for
utility and energy savings. Convenience stores, fast food
chains, and hair salons are typical locations that might
benefit from a tankless water heater.
Vacation homes are good
locations to use a tankless water heater since the heater
draws no current to heat water unless hot water is needed. If
the home is used only periodically, the tankless heater is a
good choice to save on energy costs but have hot water
available as soon as the occupants arrive.
Performance
For
a tankless water heater supplying domestic hot water, two
specifications are required to properly determine the rating
of a heater that would be adequate. The first is the
temperature rise necessary based on the inlet water
temperature expected. The second specification is the flow
rate in gallons per minute (GPM) expected for the hot water.
As an example, if the inlet
water at its coldest is 57°F (13.9°C) and the desired hot
water temperature is to be 120°F (48.9°C), the heater must
provide a 63°F (35°C) rise. The actual flow rate can vary
significantly depending on the number of hot water faucets
open at one time. A nominal value for hot water flow in a home
with average water use might be 3 GPM. However, fixtures as in
showers with multiple showerheads or whirlpool baths may
require considerably more flow of hot water.
The main concern for a
builder, installer, or inspector should be that the heater
rating and the circuits and service supply be adequate for the
installation. The necessary circuit and service rating are
treated later in this article. Since the tankless water
heaters have practically no storage capacity and they only
heat water after flow is detected, the heating elements must
be sized to meet the maximum expected water flow demand of the
house.
The tankless water heaters
are often specified by their BTU ratings and by listing the
temperature rise expected for a steady flow rate of water. As
an example, the specification for one water heater indicates
that the heater is capable of raising the input water
temperature 63°F (35°C) at a steady flow rate of 3.0 GPM
(3.78 liters/min). The electrical energy required to supply
this heater is given by this energy plus the energy lost by
the heater. Since tankless heaters are often 95 percent or
more efficient, the heating losses can usually be neglected.
As calculated in the
appendix to this article, a power input of about 28 kW is
necessary to heat the water flowing at 3 GPM to 120°F. One
model of heater has four 7000-watt elements and can thus
produce 28,000 watts at 240 volts for whole-house hot water
heating. This heater would require 117 amperes when operating
at full power.
The whole-house tankless
water heaters are available with electrical ratings from about
18 kW to 30 kW, although smaller and larger rated units are
available for special applications.
Advantages
and Disadvantages
Electric
tank water heaters are rated for storage capacity, efficiency,
and electrical demand. Typical home-use tanks hold 30 to 100
gallons of water and thus are large and are usually placed in
an area such as the garage. Electric power requirements are
determined by the rating of the heating element but a typical
tank heater could use a 5000-watt element supplied by a
30-ampere, 240-volt circuit. The efficiency of converting
electrical energy to energy in the water varies from about 60
percent but may be as high as 75 percent or higher depending
on the insulation for the tank. If the tank heater is powered
continuously but hot water is not used, a certain amount of
standby loss in energy is unavoidable. Also, if the total hot
water supply in the tank is exhausted, many minutes of
recovery time is required before hot water can be supplied
again.
In comparison with tank
heaters, the tankless heater may have a number of advantages
and disadvantages as summarized in table 1. The tankless model
described is a 28 kW version designed to supply household hot
water for a house of perhaps 2000 to 3000 sq. ft. in area with
two bathrooms.
Table
1.
The 28 kW model of tankless
heater as shown in photo 1 is about 40 x 40 x 16 cm (16 x 16 x
6-1/4 in.) and weighs only 10.5 kg (23 lbs). This size and
weight allows easy plumbing installation at any convenient
location within a home or business.
Studies have shown that a
tankless water heater is highly efficient and has practically
no standby losses.1 This can save a considerable
amount of electrical energy compared to an equivalent tank
heater, particularly if the residents use little water
compared to the average household users since the standby
losses of a tank become more significant compared to the
energy used to heat the lesser amount of water demanded at the
faucets.
A common complaint of
homeowners is that it takes too much time before hot water
reaches the faucets or showerhead in a bathroom. The wait can
be several minutes or more. Obviously, the unheated water must
be purged from the hot water pipe leading to the taps before
hot water is available. This is not only an irritation for the
user but also wastes water. One solution is to place a
tankless water heater centrally in a home so that the piping
distance for hot water to the taps is minimized. Alternatively
in a larger home, the tankless water heater can be used as
booster unit in series with a tank heater. If the tankless
heater is placed close to the taps, it supplies hot water
almost immediately and then only raises the temperature of the
water from the tank heater to overcome losses in the piping
when hot water arrives from the tank.
Perhaps the biggest
advantage of a tankless water heater is that there is no
theoretical limit to the amount of hot water that can be
produced within the flow rate capacity of the heater. For
those desiring to take very long showers, the tankless water
heater is ideal.
Although the amount of hot
water is "limitless," the flow rate that can be
sustained for water at the desired temperature is limited.
Many units advertised as whole-house units will provide up to
2–3 GPM of hot water with a 60 to 70°F temperature rise.
This should be sufficient for the hot water requirements of
one shower and a sink simultaneously. If the heater is
operating at full power and a higher flow rate is demanded, by
opening another faucet for example, the outlet temperature at
the heater will decrease. The NAHB report cited earlier
indicates that there are relatively few times that a tankless
heater of suitable rating cannot meet the demand for hot
water.1
Obviously it is difficult to
estimate prices for the purchase and installation of a
tankless water heater versus that for a tank heater. The
overall initial investment depends on the heater price and the
installation costs. However, the initial price and the cost of
the electrical circuits are generally higher for the tankless
water heater. Savings in energy costs may offset this higher
initial cost over time.
Because of the electrical
demand of a whole-house tankless water heater, some
installations may cause annoying light flicker due to the
changing voltage drop when the heater cycles between power
levels. In the author’s experience, the problem is usually
caused by an inadequately sized utility transformer or an
undersized service.
Electrical
Requirements for Tankless Water Heaters
Tankless
water heaters are referred to as instantaneous water heaters
in the NEC. Article 422 covers these heaters used for
domestic water heating. In particular, 422.11(F)(3) allows
listed instantaneous water heaters to be supplied by circuits
not exceeding 120 amperes and protected at not more than 150
amperes.2
Assuming a 240-volt,
single-phase utility system, a 28-kW tankless water heater
would require branch circuits supplying 117 amperes. A
four-element heater with 7000-watt elements could thus be
supplied by four 30-ampere circuits or two 60-ampere circuits,
or one 120-ampere circuit according to the NEC. Figure
1 shows a simplified diagram of the four circuits required for
a 28-kW heater.
Figure
1. Possible circuit configuration for 28 kW heater
In a space-heating
application, the branch-circuit conductors and overcurrent
devices are required to be rated at 125 percent of the total
heater ampere load according to 424.3(B).
Table
2 shows a sample load calculation for a 2000 sq. ft. home
with a 28-kW tankless water heater. Using NEC 220.30
for optional feeder and service calculations for a dwelling
unit, the service requirement is 200 amperes.
Various
Rules and Inspection
A
number of standards and codes that cover water heaters are
concerned with hazards due to the buildup of excessive
temperature or pressure. For example, Underwriters
Laboratories UL 499 applies to electric heating appliances.
That document and NEC 422.47 requires most water
heaters to have a temperature and pressure-relief device.
However, tankless or instantaneous heaters are exempt from
that requirement. UL 499 only requires the valve for pressure
vessels of more than 3.0 inches in inside diameter used in
heaters. Since the heaters discussed in this article would not
have heating chambers that large, no pressure relief valve is
necessary. NEC 422.47 exempts instantaneous-type water
with a capacity of 4 liters (1 gallon) or less.
When water heaters are
installed in manufactured homes, various rules apply as
defined in Part 3280—Manufactured Home Construction and
Safety Standards, issued by the U.S. Department of Housing and
Urban Development. Also, NFPA 501 covers manufactured housing.
Conclusions
A
tankless water heater can serve as an alternative to a
tank-type heater for domestic water heating or as a substitute
for a boiler in a space heating application. For further
information about tankless water heaters, see the various web
sites that compare the commercially available water heaters.3
1 "Performance Comparison
of Residential Hot Water Systems," NAHB Research
Center, Inc., Upper Marlboro, MD, November 2002.
2 Other electric heating
appliances with resistance-type heating elements must
have their heating elements subdivided to not exceed 48
amperes and be protected at not more than 60 amperes.
According to 422.13, a storage-type water heater
requires a branch-circuit rating of not less that 125
percent of the nameplate rating of the heater.
3 For example, visit the site http://www.eere.energy.gov/consumerinfo
and search for "tankless." This is a U.S.
Department of Energy site.
Thomas L. Harman is an electrical
engineer with a Ph.D. in electrical engineering from
Rice University. He has held master electrician’s
licenses in many cities and states around the U.S. and
has been a master electrician since 1973 in Houston,
Texas. He has also had considerable practical experience
in industrial, commercial and residential electrical
design and installation. Dr. Harman has been called as
an expert witness in numerous cases involving electrical
accidents and faulty electrical designs. Presently he is
the chair of the Computer Engineering department at the
University of Houston Clear Lake near NASA Johnson Space
Center in Houston, TX. He has also consulted on the
design and installation of tankless water heaters for
companies such as Microtherm, Inc.
He is a member of the National
Electrical Code code-making panel number 2 that deals
primarily with branch-circuit, feeder and service
calculations. As a result of his experience, he has
written the popular textbook Guide to the National
Electrical Code, published by Prentice Hall, Inc., Upper
Saddle River, NJ. This book is now in its ninth edition.
In private classes, Dr. Harman has used this textbook to
train many electricians to prepare for their master’s
examination.
You may reach Dr. Harman at harman@cl.uh.edu.
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